The process of ultrafiltration includes separation of solutions in colloidal systems by means of semi-permeable (ultrafiltration) membranes in special apparatuses under pressure 0.1-0.8 MPa (BEC. M: Great Russian Encyclopedia 1997, P 1249). Ultrafiltration membranes are characterized by a certain level of values of water permeability Q=(2−500)·10−4 cm/sec atm and nominal molecular weight cut-off ML=(5−500)·103 g/mol.
Thermal stability of a membrane is determined by the temperature of the beginning of destruction (loss of mass) of the polymer from which it is made.
Heat stability of the membrane is determined by a temperature of softening of the polymer from which it is made.
Non-thermostable polymeric ultrafiltration ultraporous membranes, among which there are chemically stable ones, are known and are used for purification of vaccines, blood, in the food industry, for manufacture of juices, dairy products, and for purification of sewage water. (Mulder. M. Introduction into membrane technology. M. Mir 1999. page 513; Dubiaga V. P., Prepechkin L. B., Katalevsky E. E. Polymeric Membranes. M. Khimia, 1981, page 222). For ultrafiltration, asymmetric membranes are mainly used. Known polymeric asymmetric ultrafiltration membranes, however, are limited in their use. They can not be used in aggressive conditions: at temperatures above 200° C., in aggressive media, and/or in organic solvents.
Ultrafiltration membranes are known based on soluble polyimides, since the process of their manufacture allows one to avoid the stage of imidization.
Ultrafiltration polyimide microporous membrane and method of its manufacture from soluble aromatic polyimide are known (U.S. Pat. No. 4,963,303, issued Oct. 16, 1990). For obtaining the membrane, a 14-22% forming solution is used in chlorine-containing or amide solvents of polyimide XU-218 (also known as Matrimid 5218), which contains 6-20% of poreformer caprolactam. A thin layer of the forming solution is cast on a fabric substrate and immersed into a deposition bath with water. After washing out with water, the membrane is ready to use in an apparatus for deparafinization at temperatures 100-200° C., since the membrane is thermally stable to the same degree as the initial polyimide.
U.S. Pat. No. 6,180,008, issued Jan. 30, 2001, discloses a polyimide small-pore hyperfiltration membrane from Matrimid 5218 or Lenzing P 84. For improving its service characteristics, when compared with the one described above, it includes an additional thermal treatment of membrane at 150° C., with a preliminary filling of pores with technical oil to prevent sticking.
The main disadvantage of the known membranes is that they are destroyed in solvents, in which the used polyimides are dissolved (Matrimid 5218 or Lenzing P 84). Any membrane from soluble polyimide has this disadvantage.
Ultrafiltration membranes based on soluble polyimides have thermal and heat stability, which allows one to use them at temperatures of not more than 200° C. This can be explained by the fact that the soluble polyimides contain in their main chain hinged bridge groups and/or volume groups which, as well known, provides solubility, but at the same time cause significant reduction of the level of thermal and heat stability of the polymer when compared with rigid-chain polyimides.
In other words, all ultrafiltration micro- or small-pores membranes from soluble polyimides are not completely thermally, heat and chemically stable.
A process of obtaining insoluble ultrafiltration polyimide membranes is significantly more difficult. They can not be prepared only based on rigid-chain polyimides, for example, polypyromellitimides. The forming of membranes is not possible from finished polyimide, since rigid-chain polyimides are not soluble and meltable, and it is performed by means of a forming solution of prepolymer.
A limited number of inventions are known, which are connected with an attempt to obtain ultrafiltration membranes from rigid-chain polyimides.
Japanese patent application no. 61-53086, published on Nov. 15, 1986, describes a method of obtaining a polyimide membrane based on dianhydride of 3,3′,4,4′-diphenyl tetra carbonic acid and 4,4′-diaminodiphenyl ether. The characteristics of ultrafiltration in the patent are not presented. In the known method there is a problem of preserving the porous structure obtained on the stage of deposition. During drying and subsequent thermal treatment, collapse of pores takes places.
A method of obtaining semipermeable membranes from aromatic polyimide is known based on dianhydride of 3,3′,4,4′diphenyl tetra carbonic acid and 4,4′-diaminodiphenyl ether (U.S. Pat. No. 4,378,324, published on Mar. 29, 1983). The disadvantage of this method is the use of a toxic solvent. The main application of the known membranes is gas separation, however, they can not be used for desalination of water solutions. Characteristics of ultrafiltration are not presented in the patent.
The European patent no. 0753336, published on Jan. 15, 1997, describes a polyimide membrane based on a cross-linked copolymer, which is obtained from pyromellite dianhydride, dianhydride of 3,4,4-benzophenone tetra carbonic acid and 4,4-oxydianiline. Obtaining of cross-linking the process of thermal imidization at a temperature ≧250° C. allows one probably to fix the porous structure. However, the known method, due to impossibility of control of the process of cross-linking, does not allow one to obtain membranes with a required level of reproductability of porous structure and properties of membranes.
In the technologies described in Great Britain patent no. 1,435,151, published May 12, 1976, and U.S. Pat. No. 4,113,628, published Sep. 12, 1976, the process of deposition and imidization are combined, with the use of a deposition path of a solution of imidizing mixture of acetic anhydride and triethylamine in benzole (chemical imidization). The main disadvantage of this method is the use for deposition and imidization membranes with high volumes of aggressive media of imidizing mixture, and after performing final thermal treatment of the surface layer, the membrane becomes too dense for ultrafiltration.
Other examples of known polyimide membranes are described in U.S. Pat. No. 6,716,270, published on Apr. 6, 2004. However, characteristics of ultrafiltration in the patent are not presented.
Therefore, there is a pressing need for thermally, heat and chemically stable ultrafiltration polyimide membranes suitable, in particular, for ultrafiltration.